Although officers could embark on a career in the Royal Navy, in 1819 seamen did not "join the Navy" - the Navy employed ratings on a casual basis - individual ships recruited men anyway they could: financial inducements and pressing were both legitimate methods.
On foreign stations, like St Helena, ships would frequently take men from merchant ships (whether they liked it or not). If he was rated Able Seaman on joining the Conqueror, then this meant that he was already an experienced sailor - able to reef and steer and heave the lead - normally only attained after five years or so at sea.
Men would only be employed by the Navy for the length of the commission, they were then paid-off and were free to move wherever they wished: although sometimes the Admiralty would order a ships company to be 'turned over' to a ship about to be commissioned.
For the seamen of the time, they moved constantly and easily between merchant and naval service, enjoying the freedom to choose whatever ship they wished to join, so a mixed record is quite normal and indeed the usual.
If he joined the ship at St Helena, then I would suggest that he was 'recruited' from a passing merchant vessel, and after leaving Cambrian, he went back to merchant service.
Have you checked the Pay and Muster Books for Conqueror ? - they sometimes indicate a previous ship
